<html xmlns="http://www.w3.org/1999/xhtml">
<head>

<bindings xmlns="http://www.mozilla.org/xbl"
          xmlns:xhtml="http://www.w3.org/1999/xhtml">
  <binding id="a">
    <content>
      <xhtml:div>
        <children includes="span"/>
      </xhtml:div>
      <xhtml:div>
        <children/>
      </xhtml:div>
    </content>
  </binding>  
</bindings>

</head>

<body>
<div style="-moz-binding: url(#a);">
  <div>3</div><span>1</span><div>4</div><span>2</span><div>5</div>
</div>
</body>
</html>
